Aromatic streams, especially benzene, are highly desirable and valuable chemical feedstocks for use in making plastics. Benzene can be produced by various processes. A major source of benzene is through the recovery of aromatics created in the catalytic reforming of naphtha in a petroleum refinery. Benzene can also be made through the thermal dealkylation of toluene. A high value benzene product has strict specifications of a bromine index that must be less than 20 mg/100 g, a maximum sulfur content of less than 0.2 ppm and a non-aromatics content of less than 500 ppm. During the production in refineries, benzene tends to be contaminated with olefins and diolefins which causes the benzene to miss the required specifications. Diolefins are highly unstable and reactive, causing gum formation and other stability issues. In addition, many crude oil supplies contain sulfur and the products from refining tend to contain some small amounts even with significant sulfur removal technology in the refinery. With such a low limit, meeting sulfur specifications will often be a concern and processes for producing benzene for petrochemical use must be capable of removing excess sulfur.
Current technology suggests converting the diolefins to alkanes by selective hydrogenation at temperatures between 430° F. and 500° F. using sulfided cobalt-molybdenum or nickel-molybdenum catalysts. These sulfided catalysts capture the sulfur and cause it to adhere to the catalyst separating it from the aromatic stream. Unfortunately, in this reaction, some benzene molecules are partially hydrogenated to cyclohexene and other cycloolefins which essentially reduce the benzene yields and fails to reduce the bromine index below the specifications. All of these contaminants are typically at low concentrations, but they are still problematic and substantially compromise the value of the aromatic stream.
Another technology for dealing with diolefins and sulfur includes clay treaters which are adsorbents that reduce both the sulfur content and the diolefin content without losing benzene from the product. This clay treatment process is expensive as the disposal of contaminated clay as hazardous waste must be included in operating cost and the capital cost of a clay treater is substantial for the contribution to the minimal contaminant that must be removed.
A simple and inexpensive process of removing sulfur and diolefin content from aromatics that does not create side reactions that waste aromatics and create other problem components would provide significant advantage to the financial performance of many refineries.